Add explanation for direct_assign.py failing test

tests/failing_tests/direct_assign.py

@@ -4,6 +4,18 @@ from pythonbpf.maps import HashMap
 
 from ctypes import c_void_p, c_int64
 
+# NOTE: I have decided to not fix this example for now.
+# The issue is in line 31, where we are passing an expression.
+# The update helper expects a pointer type. But the problem is
+# that we must allocate the space for said pointer in the first
+# basic block. As that usage is in a different basic block, we
+# are unable to cast the expression to a pointer type. (as we never
+# allocated space for it).
+# Shall we change our space allocation logic? That allows users to
+# spam the same helper with the same args, and still run out of
+# stack space. So we consider this usage invalid for now.
+# Might fix it later.
+
 
 @bpf
 @map